The invention relates to an improved method of and a capillary flow cell for analyzing fluid samples by microseparation techniques such as capillary electrophoresis (CE), micellar electrokinetic capillary chromatography (MECC), electrochromatography (EC), capillary liquid chromatography (LC), supercritical fluid chromatography (SFC), micro high-performance liquid chromatography (micro HPLC) and other related techniques.
CE, for example, is an important alternative and complement to GC and micro-HPLC. In use, a fluid sample is introduced into the flow cell while a UV/visible light ray beam is passed through an elongated section or sample chamber of the capillary flow cell. Light rays travelled through the elongated section are detected by a photo detector or the like and analyzed. The amount of light absorbed by the sample with time provides information about the mass and concentration of solutes contained in the sample, for example.
From U.S. Pat. No. 5,057,216 (Chervet) a bent or curved capillary flow cell is known, having an essentially Z- or U-shaped capillary tubing defining an inner lumen, and an inlet and outlet end for passing fluid samples through the lumen. Between the inlet and outlet ends an elongated section or sample chamber is formed, which connects to a first and second light transmittive bend of the tubing. The elongated section has a longitudinal axis and is positioned in a bore of a template such that the bends extend on either side of the template.
In use, a fluid sample is transported through the lumen of the capillary tubing while a light ray beam as much as possible parallel with the longitudinal axis of the elongated section is incident on the first bend. Light travelled through the elongated section and emanating from the second bend is detected by a detector and analyzed.
A bent capillary flow cell of this type, integrally formed from one piece of tubing, is essentially free of dead volumes. Further, any contact between the fluid sample to be analyzed and light source means and detector means is prevented, because these means are positioned outside the flow cell near its light transmittive bends. This is different from the flow cell disclosed by European patent application 0,089,157 wherein optical waveguides are used for light transmission to and from the elongated section or sample chamber. These optical waveguides, i.e. optical fibers, are fused into the open ends of the chamber to which further inlet and outlet means connect, for passing a fluid sample through the sample chamber.
In the flow cell according to EP-A-0,089,157 the optical waveguides are in direct contact with the fluid sample which can have, inter alia, a detrimental influence on the reliability and accuracy of the analysis because of the possibility of chemical interaction with the fluid.
Further, the connection of the waveguides and the inlet and outlet means to the sample chamber is very cumbersome. This not only because of leakage problems but also in that relatively large dead volumes near these connections are unavoidable. These dead volumes lead to turbulence in the fluid and cause undesired dispersion effects. Because of the risk of fluid leakage, with the use of high voltages in CE of typically 30,000 to 50,000 Volts, safety aspects can not be sufficiently guaranteed for flow cells of the type having fused connections according to EP-A-0,089,157.
Despite its advantages, in particular for analyzing some types of fluids, the signal-to-noise (S/N) ratio of the bent capillary flow cell according to U.S. Pat. No. 5,057,216 is too low for accurate measurement.